The present invention relates to magneto-optical recording method for recording information on a magneto-optical recording media and a recording apparatus used therefore, and more particularly to a magneto-optical recording method for overwriting a magneto-optical disk by a modulated optical beam and a recording apparatus used therefore.
Magneto-optical recording media such as rewritable magneto-optical recording disks are recorded with information by means of a modulated laser beam in the form of recording mark. The recording mark typically has a sub-micron size. The information thus recorded on the disk is read out therefrom also by a laser beam. Thereby, a significant increase in the recording capacity is achieved over conventional recording media such as flexible magnetic disks or hard disks. Thus, intensive efforts are being made on magneto-optical recording apparatuses to realize largecapacity, external storage device of computers.
For example, a magneto-optical disk having a diameter of 3.5 inches typically has a storage capacity of about 128 Mbytes on each side. On the other hand, flexible magnetic disks of the same diameter can provide only about 1 Mbytes of storage capacity on each side. This means that a single magneto-optical disk can provide storage capacity comparable to the storage capacity of about 128 flexible magnetic disks. In addition, the magneto-optical disks are replaceable similarly to flexible magnetic disks and provides distinct advantage over fixed storage devices such as hard disk.
On the other hand, magneto-optical disks generally have a drawback in the point that data transfer is slow. For example, a data transfer rate of about 640 kB/sec is common for magneto-optical disks revolved at a speed of 2400-3000 rpm, while a data transfer rate of about 3 Mbytes/sec is achieved in the hard disks revolving at a speed of 3600 rpm. The reason of this undesirable result is attributed to the recording process adopted in the magneto-optical disk devices. It should be noted that overwrite recording of information, generally employed in the hard disk devices, cannot be employed in the conventional magneto-optical disk devices. In order to write information on a recorded disk, one has to erase a recording track before starting recording of information. Thereby, the disk is required to revolve for one turn to achieve erasing and another one turn for verifying. Obviously, the process for erasing track is extraneous and decreases the data transfer rate.
In order to avoid the foregoing problem, efforts have been made to develop magneto-optical disks that allows overwrite recording. In order that such overwrite magneto-optical disks are accepted in the society, it is necessary to reduce the circuits that are added to the currently used system as much as possible to reduce the size as well as the cost of the recording apparatus.
Conventionally, so-called magnetic modulation process and optical modulation process are developed as candidate processes for overwriting a magneto-optical disk. In the magnetic modulation process, a magneto-optical disk having a single layer of magnetic coating is employed, and overwrite recording is achieved by means of a magnetic head while irradiating the magneto-optical disk continuously by a laser beam. In this process, therefore, the magnetic field applied to the magneto-optical disk is selectively inverted in response to recording information by the magnetic head. Thus, the speed of overwrite recording in this process is limited by the speed of magnetic inversion in the magnetic head.
On the other hand, the optical modulation process uses a magneto-optical disk that carries thereon a plurality of magnetic coatings, and the overwrite recording is achieved by changing the intensity of the laser beam. In this process, therefore, the speed of overwrite recording is determined mainly by the modulation speed of the laser beam intensity and one can obtain a superior operational speed over the magnetic modulation process.
Hereinafter, the principle of the overwrite recording according to the optical modulation process will be described with reference to FIG. 1 that shows the cross sectional structure of an exchange-coupled layered recording medium. As shown in FIG. 1, the recording medium is constructed on a disc-shaped, transparent substrate 1 and includes a protective layer 2, a memory layer 3, an intermediate layer 4, a recording layer 5 and another protective layer 6, wherein the layers 2-6 are deposited consecutively on the substrate 1. Thereby, the layers 3-5 form a three-layered recording structure. FIG. 2 shows the temperature-versus-coercive force characteristics of the memory layer 3 and the recording layer 5, wherein the characteristic for the memory layer 3 is designated as I and the characteristic for the recording layer 5 is designated as II. As indicated in FIG. 2, the memory layer 3 has a Curie temperature Tc that is lower than the Curie temperature Tc of the recording layer 5. On the other hand, the memory layer 3 has a coercive force Hc that is larger than the coercive force Hc of the recording layer 5 at the room temperature. The intermediate layer 4 is provided to control the exchange coupling between the memory layer 3 and the recording layer 5. Further, FIG. 2 shows the initializing magnetic field H.sub.ini to be described later.
Next, the conventional overwriting process will be described. For the sake of simplicity, the erasing process and the recording process are described separately. When erasing, the magneto-optical disk is applied with the initializing magnetic field H.sub.ini such that the magnetization is aligned in the direction of the magnetic field H.sub.ini as indicated schematically in FIG. 3. During this process, the magnetization of the memory layer 3 is not affected, as the magnetic disk is held in the room temperature environment and the portion of the magneto-optical disk that is applied with the magnetic field H.sub.ini is also held at the room temperature.
Next, the magneto-optical disk is applied with a bias magnetic field H.sub.b in correspondence to the portion that has been applied with the initializing magnetic field H.sub.ini, and a laser beam 8 scans the surface of the magnetic disk with a first, reduced optical power P.sub.L. Thereby, the memory layer 3 is heated to a temperature that causes an inversion of magnetization. On the other hand, this temperature is below the Curie temperature Tc of the recording layer 5, and no inversion of the magnetization occurs in the recording layer 5.
As a result, the portion of the memory layer 3 that is irradiated by the laser beam experiences inversion of magnetization as a result of the magnetic exchange coupling with the recording layer in coincidence to the direction of magnetization of the recording layer 5, and erasing of information is achieved thereby. In FIG. 3, P.sub.1 represents the region wherein erasing has been achieved and P.sub.2 represents the region wherein previous recording of information has not been erased yet.
Next, recording of information will be described with reference to FIG. 4. As indicated in FIG. 4 schematically, a laser beam 8' having an increased optical power P.sub.H is produced and radiated upon the magneto-optical disk. In correspondence to the portion of the magneto-optical disk that is irradiated by the laser beam 8', it will be noted that a bias magnetic field H.sub.b is applied. Further, in correspondence to the portion of the magneto-optical disk that precedes the portion of optical irradiation, an initializing magnetic field H.sub.ini is applied in the direction opposite to the direction of the bias magnetic field Hb.
As the laser beam 8' is set to have a high optical power P.sub.H, the recording layer 5 experiences a temperature elevation above the Curie temperature in correspondence to the portion irradiated by the laser beam, and the portion of the magneto-optical disk that has been irradiated by the optical beam 8' is magnetized in the direction of the bias magnetic field Hb. In addition, the memory layer 3 experiences a temperature rise above the Curie point and magnetized in the direction of the bias magnetic field Hb. Thereby, recording of information is achieved. In FIG. 4, the region designated as P.sub.3 represents the region wherein the recording of information has been achieved while the region designated as P.sub.4 represents the erased region wherein recording has not been achieved yet. When recording information, the laser beam is subjected to an amplitude modulation such that the optical power of the laser beam is set to the foregoing high optical power P.sub.H in correspondence to logic value "1" of the recording data and to the low optical power in correspondence to the logic value "0."
In the actual overwrite recording process, a recording is achieved on the portion of the recording medium that has been erased previously with the low optical power P.sub.L, by means of the laser beam having the high optical power P.sub.H as indicated in FIG. 30. In the illustration of FIG. 15, it should be noted that P.sub.R represents the optical power used for reading.
As another recording method that uses an optical modulation process, there is a proposal to use a magneto-optical disk having a four-layer structure that includes a magnetic layer acting as the initializing magnet (Nikkei Electronics, 1990. 8. 6., pp. 173-180). Thereby, one can omit the initializing magnet.
On the other hand, the foregoing conventional recording process has a drawback in the point that the optimum recording power may change depending on the recording pattern. Hereinafter, this problem will be examined in detail with reference to the case of recording data in the form of (2, 7) run-length-limited modulation process designated hereinafter as (2, 7)RLL process. In the (2, 7)RLL process, a data word of m bits and having a bit interval .tau. is converted to an RLL code word of 2 m channel bits, wherein m=2, 3, 4.
It should be noted that RLL code is formed according to a rule such that two adjacent channel bits having a logic value "1" are separated from each other by at least two, but smaller than eight digits of data "0." Thus, the interval between two adjacent recording marks becomes minimum when the RLL format of the recording data has a three channel-bit interval (1.5.tau. signal), wherein two adjacent channel bits "1" are separated by two bits of logic value "0." On the other hand, the interval between the recording bits becomes maximum when the RLL format of the recording data has eight channel-bit interval (4.tau. signal) wherein two adjacent channel bits "1" are separated from each other by seven bits of logic value "0."
In the pit-position recording method known also as mark interval recording method, the magneto-optical recording medium is recorded with a recording mark (pit) in response to the logic value "1" of the RLL signal, while the recording of the mark is suppressed in correspondence to the logic value "0" of the RLL signal. Thereby, recording marks are recorded on the magneto-optical disk in the form of recording mark as indicated in FIGS. 6(A) and 6(B), wherein FIG. 6(A) shows the pattern corresponding to continuous recording of the 1.5.tau. signal while FIG. 6(B) shows the pattern corresponding to continuous recording of the 4.tau. signal.
FIG. 7 shows the C/N map indicative of an iso-C/N ratio contour line for various combinations of the setting for the high optical power level P.sub.H and the setting for the low optical power level P.sub.L. The result shown in FIG. 7 is obtained for a magneto-optical disk revolving at a line velocity of 9 m/sec, by recording a 1.5.tau. signal continuously after erasing and further overwriting the 1.5.tau. signal by a 4.tau. signal as indicated by broken lines III-1 and III-2. Further, FIG. 7 shows the result obtained for the same magneto-optical disk revolving at the same speed, by recording a 4.tau. signal after erasing and overwriting the 4.tau. signal by a 1.5.tau. signal by continuous lines IV-1 and IV-2.
As indicated in the broken lines III-1 and III-2, one has to set the optical power P.sub.L to have a value of about 3.5-5 mW and the optical power P.sub.H to have a value of about 7 mW or more, in order to obtain a practical C/N ratio of 45 dB or more when overwriting the magneto-optical disk with the 4.tau. signal.
On the other hand, as indicated in the continuous lines IV-1 and IV-2, it is necessary to set the optical power P.sub.L to be less than 3 mW and the optical power P.sub.H to have a value of about 8 mW, in order to achieve the practical C/N ratio of 45 dB or more when overwriting the magneto-optical disk with the 1.5.tau. signal. The result of FIG. 7 indicates that the optimum combination of the optical power P.sub.L and P.sub.H changes depending upon the recording data, and that it is difficult to obtain satisfactory recording of data as long as the optical power level P.sub.L and the optical power level P.sub.H are fixed. This problem of variation of the optimum optical power when recording the 1.5.tau. signal and when recording the 4.tau. signal becomes particularly conspicuous with respect to the setting of the optical power P.sub.L.
The foregoing result that the optimum setting of the low optical power P.sub.L changes depending on the recording pattern is believed to be caused by the fact that one needs to use a large optical power in order to heat the magneto-optical recording medium to a temperature wherein erasing of information occurs, provided that the recording marks are separated form each other by a large interval as in the case of the 4.tau. signal. When the interval between the recording marks is small as in the case of recording the 1.5.tau. signal, on the other hand, the diffusion of heat from the part of the magneto-optical disk irradiated by the optical power P.sub.H, induces a temperature rise and a small optical power is sufficient for the optical power P.sub.L to cause the desired erasing. In addition, the recording of pit on the recording medium with high optical power beam causes erasing of information in correspondence to the part located adjacent to the pit.
FIGS. 8(A) and 8(B) explain the foregoing principle. When a magneto-optical disk is irradiated by a laser beam having a high optical power P.sub.H in correspondence to an interval between t.sub.1 and t.sub.4 as indicated in FIG. 8(A), the temperature of the magneto-optical disk rises in the irradiated portion as indicated in FIG. 8(B), wherein an erase temperature T.sub.PL is reached at a timing d.sub.2 that is immediately after the timing t.sub.1. The temperature rises further and reaches a write temperature T.sub.PH at a timing d.sub.3. The temperature further rises after the timing d.sub.3 and only to start decrease in correspondence to the timing t.sub.4. Thus, the foregoing write temperature T.sub.PH is attained at the timing d.sub.4 and the erase temperature T.sub.PL is reached in correspondence to the timing d.sub.5.
Thus, recording of pit (recording mark) is achieved in correspondence to the interval between the timing d.sub.3 and d.sub.4 in which the temperature of the recording medium is higher than the write temperature T.sub.PH. Further, erasing is achieved in correspondence to the intervals d.sub.2 -d.sub.3 and d.sub.4 -d.sub.5 located before and after the foregoing interval d.sub.3 -d.sub.4. Particularly, it should be noted that the erasing is achieved in correspondence to the interval d.sub.4 -d.sub.5 without irradiating laser beam. Thus, when recording data characterized by very small interval between the recording pulses, it is even possible to erase the previous recording simultaneously to the recording, without irradiating low optical power laser beam. The foregoing analysis clearly indicates that the optimum value for the optical power level P.sub.L changes depending on the recording pattern.
As such, it is necessary to construct an optical recording apparatus such that the laser power can be changed variously depending upon the recording data such that the laser optical power is held optimum. However, such a control of the optical power of the laser diode requires multiple-level control so as to control the optical power in more than three levels while such a multiple-level control requires a complex control circuit that increases the size and cost of the recording apparatus.